This invention relates to gas flow measuring devices. More specifically, this invention relates to gas flow measuring devices for providing electrical signals representative of gas flow.
Many techniques exist in the prior art for the electronic measurement of gas flow. Typically, such techniques require prior knowledge, of the physical properties of the gas to be measured so that the measuring instrument can be calibrated. Typical examples of such physical properties are the specific heat capacity, density, viscosity, and thermal conductivity of the gas. Many of these physical parameters are themselves dependent upon the composition of the gas. For applications in which the gas composition is unknown or variable in an unpredictable way, such techniques are both inaccurate and unsuitable.
Additional disadvantages inherent in many prior art techniques include limitations in the range of flow rates over which accurate measurements can be reliably made, sometimes termed the "turn-down ratio", and the generation of unacceptable back pressure in the gas path when the flow meter is connected and operational.
In an attempt to overcome the above-noted shortcomings of the prior art, flow meters have been developed which are less sensitive to gas physical parameters. This class of flowmeter is generally termed a positive displacement type flowmeter. In a positive displacement flowmeter, the gas whose flow is to be measured is periodically accumulated in a separate confining chamber, such as a cylinder with a tight fitting piston, whose volume increases at a rate equal to the flow of the gas (e.g., by displacement of the piston). By measuring the rate of increase of the confined volume (e.g., by generating a signal representative of the amount of displacement of the piston), the gas flow rate may be computed. While specific flowmeter details (such as the manner of defining the confined volume and the technique for transducing the rate of volume change) differ depending on the style and application of a given positive displacement flowmeter, all such flowmeters possess the desirable property of reasonable accuracy in the gas flow measurement independent of the physical parameters of the gas being measured. However, a severe disadvantage in known positive displacement flowmeters is the disturbance in the gas flow caused by the necessity of displacing the movable surface of the confining chamber in order to generate the measurement signal. This introduces back pressure characteristics which render such flowmeters unsuitable for use in any application sensitive to the periodic introduction of gas flow back pressure.
One positive displacement type gas flowmeter which has been developed to reduce the adverse back pressure effect is the soap-film type of positive displacement flowmeter. In this type of flowmeter, the confined volume is defined by a smooth-walled cylindrical tube typically fabricated from transparent glass. The tube has an inlet end connected to the gas whose flow is to be measured and an outlet end open to ambient. Adjacent the inlet end is an arrangement for introducing a soap film to be swept along the inner volume of the tube by the advancing gas flow. Optical sensors arranged at predetermined locations along the tube measure the transit time of the soap film translated along the interior of the tube, and this time value is converted to a gas flow rate using a known algorithm. Due to the fact that only the relatively small forces of viscosity and surface tension effects oppose the motion of the soap film and thus the flow of the gas stream, the back pressures generated by this type of flowmeter are generally sufficiently low to be acceptable. In addition, the soap film flowmeter has a dynamic measurement range which is substantially broader than that of other positive displacement flowmeters (typically in the range from about 0.5 to about 500 MI/min.). Also, this type of flowmeter is relatively inexpensive to manufacture., and maintain. The major disadvantage of the soap film flowmeter is the requirement that soap film must be periodically generated, introduced into the gas stream and somehow exhausted. Generation of the soap film is typically performed by means of a manually operated bubble generating device, which requires the presence of a human operator and prevents automatic or unattended operation. Also, due to the fact that the wet film is introduced into the gas stream, the stream is contaminated with water vapor, which is unsuitable for many in-line applications involving gasses.